Err... no. If you're underweight, you are undernourished. If you're undernourished, you lack the correct amounts of at least some nutrients that are required

Err... no. If you're underweight, you are undernourished. If you're undernourished, you lack the correct amounts of at least some nutrients that are required for proper bodily function. Being underweight can cause the following issues (and others): a severely weakened immune system, increased chance of early-onset osteoperosis (sort of similar to the increased chance of early-onset dementia), heart irregularities which can lead to serious heart damage, and anemia. Edit: Now that you've edited your post to include the dangerous components of being underweight, I don't really see how you can support your conclusion.